Electrode joints



Sept. 10, 195 E. c. THOMAS ELECTRODE JOINTS Filed Aug. 6, 1954 3 E. E Mm F nited States Patent O M nrncrnona roiNrs Edward C. Thomas, Wilmette,111., assignor to Great Lakes Carbon Corporation, New York, N. Y., acorporation of Delaware Application August 6, 1954, Serial No. 443,323 7Claims. (Cl. 287-127) This invention relates to an improved joint forcarbon electrodes. More particularly, this invention relates to animproved nipple for joining carbon and graphite electrodes used inelectrothermal furnaces.

The use of carbon electrodes, which may be either amorphous or graphiticin nature, in electrothermal processes is well established. For example,electrodes consisting of amorphous carbon are widely used in themanufacture of calicum carbide as well as in the manufacture offerroalloys and other alloys in which the carbon content is notcritical. Graphite electrodes find particular utility in the manufactureof special alloy steels. In either event, the product to be manufacturedis subjected to a high temperature in an electrothermal or metallurgicalfurnace, the heat in said furnaces being supplied by passing an electriccurrent of high amperage through an air space between the two or morecarbon electrodes or between the electrodes and the charge. Theresultant high-temperature arc produces the melting, smelting or otherelectrothermal action desired.

Graphite electrodes for use in electric furnaces are commonly joined toform a continuous column as they are consumed, by a threaded, graphite,double male member, known in the art as a nipple and similar in functionto that of a dowel pin. The nipple, of necessity, has a reduced crosssection compared with the column of electrodes it supports. Accordingly,there is a stress concentration within the body of a nipple joining twoelectrodes, which demands that the mechanical strength of the nipplestock be greater than that of the material within the body of theelectrodes it joins. As a consequence of this, nipple stock must be madeof cokes known to produce stock of higher mechanical strength anddensity. This is done by select mix formulation and usually subsequentimpregnation (after baking) with suitable impregnants. Either or all ofthese strengthening mechanisms have. been found to increase thetransverse (radial) thermal expansion characteristics of the nipplestock over and above that found in electrode stocks. As a result, undueradial pressure is exerted on the threaded female electrode socket bythe nipple when the assembled joint becomes heated to the temperaturesexperienced during normal use. On withdrawal from the heated furnace thethermally prestressed socket becomes chilled due to radiant loss ofheat. it then shrinks, and since it is already prestressed in tension,splitting of the socket often occurs.

Attempts have been made to solve socket-splitting problems by cuttingradial or diametrical kerfs or slots in aportion of the nipple paralelto its vertical axis, the purposebeing to allow radial or diametricalcompression of the nipple into the slotted region and thus obviate unduethermal expansive forces on the electrode socket. However, since theexpansion forces operate along every diameter throughout the length ofthe nipple such slot ting does not eliminate the problem of socketsplits.

--' -Slotting procedures have also been applied to electrodes toreducespalling (U. S. 2,527,294-B. L. Bailey;

P 2,805,879 Patented Sept. 10, 1957 and U. S. 2,603,669C. H. Chappell).Here, however, the problem involved is wholly different from that ofsocket splitting discussed above. Socket splits result from thedifferential radial thermal expansion characteristics in the electrodejoint, i. e. between the nipple and the socket. For example, in allowinga heated electrodejoint assembly to cool between electrothermaloperations, the socket walls which had become stressed in tension in thehot furnace by the aforementioned excessive diametrical expansion of thenipple start to shrink due to radiant cooling of the outer surface ofthe joint. As a result of this shrinkage which is aded to the tensilestress generated when the joint became heated in the furnace, tremendoushoop stresses are set up within the socket which often cause the socketwall to crack. These hoop stresses are much greater, and have anentirely different origin than the cooling stresses which result in thecondition referred to as electrode spalling. Spalling in electrodes isnot restricted to the region of the joint but rather is a surfacephenomenon throughout the length of the electrode. ttempts have beenmade to reduce this by cutting internal and external radial,longitudinal slots into the electrodes. The function of the internalslots is to dissipate contractile forces in the outer surface of theelectrode by permitting movement of the side walls of each slot towardeach other. While such means may reduce crack propagation in electrodespalling, it would not relieve the substantially greater hoop stresseswithin the socket. Rather, it would appear that such slots would providea starting point for the splitting of a socket subject to hoop stresses.It is further to be observed that such slots would reduce the physicalcross section of the socket wall, which is already being over stressed,thereby weakening the socket mechanically.

It is an object of this invention to provide improved carbon nipples foruse in joining amorphous carbon and graphite electrodes.

It is a further object of the invention to provide improved nippleswhich will not fracture the electrodes with which they are used whensubjected to thermal and mechanical shock.

it is a further object of the invention to provide a continuouselectrode system that is less subject to thermal and mechanical fractureat the electrode joints.

The above objects as well as others which will become apparent upon morecomplete understanding of the invention as subsequently herein describedare accomplished by manufacturing a nipple having at least threenonradiaLlongitudinal kerfs or slots as set forth in the followingdescription. 5

Reference should be made to the accompanying drawings whichdiagrammatically illustrate a number of embodiments of the invention.

Figure 1 is a perspective view of a nipple it), having three kerfs 12intersecting the threads 11.

Figure 2 is a cross-sectional view of the nipple shown in Figure 1 takenalong the line 2-4.

Figure 3. is a similar cross-sectional view of a further embodiment ofthe invention wherein four kerfs are used. 7 Figure 4 is across-sectional view of a further embodiment of the invention whereincurved kerfs 13 are used.

Figure 5 is a cross-sectional view of a further embodiment of theinvention having a single kerf 1'4 of Archimedean form.

It has been found that by placing one or more nonradial, longitudinalkerfs or slots in a nipple used to join carbon or graphite electrodes,the resultant joint exhibits markedly improved resistance to mechanicaland thermal shocks to which such joints are subjected in electrothermalprocesses. These kerfs may be placed in the graphite nipples by asawingoperation takingcareto avoid seve'rin'g any substantial por'tionof the nipple (either a pcrtptreral 'Oi'bififa'l pbriiOifl';

A broad embodiment of the invention comprises preparing. a nipplebymixingtogether carbon aggregateof sn'chicarbon's or carbon producingmaterials as calcined petroleum coke, anthracite, graphite, electrodecarbon scra orthefike' with a suflicienta'mount of pitch or'bitnnnnousmaterialto serve as a binder. The'mixing. operation is usually conductedabovethe melting point of the binder in order to insure uniformdistribution of the binder throughou't'th'e carbon aggregate. Smallamounts o'f viscou's petroleum oils may'be added as a lubricant,especially in the event that the nipple stock is to be prepared byextrusion througha' die. The extruded or' molded. greennip'ple stock isbaked inorder to carb'onize' the binder after which the bakedcarbon bodymay be: impregnated with a suitable impregnant after which it may beheated to'higher temperatures in order to graphitize the carbon, thecarbonized impregnant and the-carbonized binder which comprises thenipple stock. The nipple stock may then be machined into a threadednipple having either; a'cylin'drical form or the tapered formillustrated in Figure 1. At least three non-radial slots-or kerfs arecut into the nipple in the longitudinal direction with respect to thenipple axis. The depth of the slots is more than 40% of the majordiameter of the nipple but in any case substantially short of severing aportion of the nipple.

In a preferred embodiment of the invention, a threaded graphite nippleis prepared as described above. Three nonradial, longitudinal kerfs arethen cut into the nipple in such a manner as to form a symmetricallinear pattern in any cross section, such as the pattern illustrated inFigure 2. The depth of the kerfs is more than 40% of the major diameterof the nipple, but in any case is substantially short of severing anyportion of the nipple.

In a further'em'bodim'entof the invention the threaded nipple preparedas described above has four nonradial, longitudinal slots or kerfs cutinto the nipple so as to form a symmetrical linear pattern in anytransverse cross section of the nipple, each of the kerfs being in aplane perpendicular to the planes of the two adjacent kerfs. Thisembodiment is illustrated in Figure 3.

In a further embodiment of the invention at least three curvilinearkerfs arecut into a threaded nipple prepared as described above, so asto form curvilinear patterns in a transverse cross section of thenipple, the kerfs being of a depth more than 40% of the major diameterof the nipple but in-any case substantially short of severing a portionof the nipple. This embodiment is illustrated in Figure 4. V I

In a further embodiment of-the inventionacurvilinear nonradia l,longitudinal kerf or slot is cut into the nippleprepared as describedabove so as to form an Archimedean spiral in a transverse cross sectionof the nipple. Such kerfs will naturally have a depth more than 40% ofthe major diameter of the nipple and in any case will not sever aportion of the nipple. A spiral kerf such as this one, illustrated inFigure 5, has been found to be equivalent to the three or more kerfsdescribed in the embodiments above.

Modification-s of the above embodiments, which are also considered apart of this invention, include variations in which the cross-sectionalpattern formed is nonsymmetrical and/or combinations of linear (straightlines) and curvilinear patterns. I

In all of the embodiments described above, width of the kerfs may'varyfrom about 0.035 inch to 0.125 inch; In some cases it has been foundadvantageous to place a cardboard shim" or equivalent within the kerfs;Such shims will end greater rigidity to' the nipple-for purposes'ofassembly. Subsequent use of the nipple in an electrode joint duringelectrothe'rmal operations will result in shrink age-andcarbonizationof-the shim, so that there will be little remaining toprevent movement of the sidewalls of the kerfs toward each other toachieve their assigned flilfiiiii'rf of'stis'rlief. F illifi'g' thekerrswith' a carbons ceous binder material (which is essentially a solidat room temperature) prior to assembly will serve the same purpose asthe cardboard shims.

In order to more fully illustrate the nature and character of theinvention, but with no intention of being limited thereby, the followingexamples are set forth. Examples I through V are representative of theprior electrode art and reasonable modifications thereof.

Example I A threaded graphite nipple (approximately 11" diameter)similar to that illustrated in Figure 1, but not slotted, was used tojoin two graphite electrodes (20" diameter) having appropriatelythreaded sockets. The assembly was lowered into a Globar furnace andheated to a temperature of 1390" C. The assembly was then removed fromthe furnace and allowed to cool suspended in the open air. One electrodesocket split after 3 /2 minutes of cooling.

Example II A-nipple-such as that described in Example I was'sawed so asto provide a single longitudinal radial kerf of a depth of 60% ofthe-major diameter and a width of 0.035 inch.- The' nipple was used tojoin two electrodes as described above and the assembly was similarlylowered'into the furnace'and heated to 1450 C. On removal to cool, asdescribed in Example I, no splits developed. The assembled jointwasreturned to the furnace and heated to 1505 C. and again removed to cool.After 4 minutes o'f cooling, a split developed in one socket;

Example III A nipple such as'th'at described in Example II but-havingakerf width of 0.125 inch was tested as described above heating'to 1450"C. No socket split developed. On reheating to 1505" C. a split developedafter'5 /z minutes of'cooling.

ExampleIV A nipple such as that described in Example I was sawed so asto provide three radial longitudinal kerfs having a depth /2 the majorradius or about 2% inches anda kerf of 0.035- inch. When tested to 1480C. as described above, a socket splitdeveloped after 5 /2 minutes ofcooling.

Example V Anipple such as that described in Example I was sawed so as toprovide three nonradial longitudinal kerfs of a depth -Va'the radius andhaving a kerf width of 0.035 inch. When-tested at 1480- C. as describedabove, no socket splitsneveleped. The assembly was then heated to -1520'C; On coolinga; socket split developedafter- 3 36P- minu't'es;

Example VI A nipple su'ch a's'that described in Example Twas-sawed soas" to' provide three nonradial longitudinal kerfs of a' depth 64% ofthe 'major diameter of the nipple and havingfa'kerf'width' of .035 inch.The cross-sectional pattern w'as'similar' to that'illustrated in Figure2. Upon heating the' electrode assembly and cooling, as described above;

no socket splits developed.

2 Example VII A nipplesuch-as that described in Exampl'e Vl-ex'cept thatthe kerf width' was 0.052 inch, was used in' anelectrode'assembly whichwas heated to'1600" C. On" 0001 ing; nosock'et splits 'developed.

Example VIII A nipplesuch-asthat described in ExampleVI, except that thedepth-ofthe kerf was 57% of the major diameter, was used in an-electrodeassembly which was heated to-1550 C. Oncooling, no socket spli-tsdeveloped Example 1X A nipple such as that described in Example VIIIexcept that the kerf width was .052 inch, was used in an electrodeassembly which was heated to 1550" C. On cooling, no socket splitsdeveloped.

From the results obtained in the above examples, it can be seen thatelectrode joints having nipples with at least three radial, longitudinalkerfs of a depth more than 40% of the major diameter of the nipple butin any case substantially short of severing a portion of the nipple, canbe carried to considerably higher temperatures, such as thoseencountered in commercial electrothermal furnace operations, withoutdanger of developing socket splits.

Various embodiments of the invention were represented by Examples VI,VII, VIII and IX. In each case, socket splits were avoided on coolingafter heating to temperatures of 1550 C. and higher. The other examples,representing the prior electrode art, all developed socket splits oncooling after heating to temperatures ranging from 1390 to 1520 C.

The word carbon as used herein and in the appended claims is intended toinclude both the so-called gasbaked or amorphous carbon and alsographite.

Having thus described my invention, I desire it to be understood that myinvention is not to be limited other than as defined by the claims.

I claim:

1. A carbon nipple for joining carbon electrodes, having at least threenonradial, longitudinal kerfs extending the entire length of the nippleand of a depth more than 40% of the major diameter of said nipple but inany case substantially short of severing a portion of said nipple.

2. A nipple according to claim 1 having three kerfs, said kerfs forminga linear pattern in a transverse cross section of said nipple.

3. A nipple according to claim 1 having three kerfs, said kerfs forminga symmetrical linear pattern in a transverse cross section of saidnipple.

4. A nipple according to claim 1 having four kerfs, said kerfs forming alinear pattern in a transverse cross section of said nipple.

5. A nipple according to claim 1 having four kerfs, said kerfs forming asymmetrical linear pattern in a transverse cross section of said nipple,each of said kerfs being in a plane perpendicular to the planes of thetwo adjacent kerfs.

6. A nipple according to claim 1 in which said kerfs form curvilinearpatterns in a transverse cross section of said nipple.

7. A carbon nipple for joining carbon electrodes, having a longitudinalkerf extending the entire length of the nipple and forming anArchimedean spiral in a transverse cross section of said nipple, saidspiral having its terminus at the approximate center of said crosssection.

References Cited in the file of this patent UNITED STATES PATENTS1,287,678 Hall Dec. 17, 1918 2,125,018 Hamill July 26, 1938 2,482,176Hamister Sept. 20, 1949 2,527,294 Bailey Oct. 24, 1950

